Ceramic materials exhibiting such favorable properties as high hardness, wear and corrosion resistance, electrical resistivity, and low friction, have traditionally been used at relatively low temperatures, or under low tensile stress at high temperatures. Recently, however, major efforts have been made to use ceramics in structural applications that include a combination of conditions including significant tensile or flexure stresses, a range of temperatures, and corrosion environments. The ceramics that have being developed to meet these needs are generally referred to as technical, advanced or high performance ceramics. Materials of this type are generally produced by state-of-the-art technology and are therefore also known as high technology ceramics.
It is well recognized in the ceramics industry that one of the key technologies that will enhance the use of high technology ceramics is the ability to reliably (i) join simple shape components to form complex assemblies, (ii) join unit lengths of materials to form large systems, and (III) join high technology ceramic components to metals.
In an effort to join such ceramics, active metal brazing has been developed resulting in simplified processing while preserving or enhancing bond integrity. Active metal brazing is a technique where wetting and bonding of the base braze material is improved due to the presence of a small amount of active metal such as titanium or zirconium.
It is, however, well known in the art that the thermal expansion mismatch between many materials, particularly a metal and ceramic member, requires the use of a ductile base braze material. Thus, brittle brazing materials, such as the gold-based alloys disclosed in U.S. Pat. No. 4,447,391 (containing boron), have limited usefulness.
However, the gold-nickel-titanium brazing alloys disclosed in U.S. Pat. No. 4,938,922 are illustrative of ductile brazing materials. In order to achieve the desired ductility the titanium content of these alloys is generally maintained at very low levels, i.e. 0.1% to 2.0% weight percent. The disclosed gold-nickel-titanium alloys may be employed in a single step process which produces a highly ductile brazed joint with excellent oxidation resistance at 650.degree. C. and no visible reaction to acid and alkali treatment. The alloy has been successfully employed to braze silicon nitride ceramic to Incolloy 909 alloy for use in internal combustion engines, and is currently produced and sold by The Morgan Crucible Company plc, Wesgo Division, under the trademark SNW-3000.
The gold-nickel-titanium alloys of the noted patent will exhibit sufficient wetting to an alumina ceramic, but, over a very narrow temperature range of about 20.degree. C. Exceeding this temperature results in dewetting--that is, the molten alloy beads up leaving bare ceramic where molten alloy formerly coated the surface.
It has, however, been found that if vanadium is substituted as the active metal in the brazing material, such as the titanium containing alloys disclosed in U.S. Pat. No. 4,938,922, higher quantities of vanadium may be employed while still maintaining the desired ductility. The increased quantity of active metal, in this case vanadium, will also significantly enhance the wetting characteristics of the brazing material over a broader range of temperatures.
Illustrative are the gold-based alloys disclosed in U.S. Pat. No. 4,606,978, containing gold, nickel, vanadium and, optionally molybdenum to prevent high temperature creep. The gold content for the disclosed alloys is generally maintained at approximately 25-85% by weight. The molybdenum content for these alloys is, however, maintained at high levels, i.e. 6-40% by weight.
What has, however, recently been found is that the addition of very low levels of molybdenum to gold-nickel-vanadium brazing materials, such as those disclosed in U.S. Pat. No. 4,606,978, will significantly enhance the ductility of the brazing material. The brazing material will therefore exhibit a lower yield strength. The lower yield strength results in lower residual stress in a brazed joint since the plastic deformation of the brazing material accommodates the thermal expansion mismatch between articles being brazed. The brazing material will also be easier to mechanically reduce/deform (i.e., rolling processes), minimizing any edge cracking which is generally associated therewith.
It is therefore an object of the present invention to provide a brazing material that can be directly brazed to a ceramic surface over a broader range of temperature and yield a highly ductile, oxidation and corrosion resistant brazed joint.